The long-term goal of this research is to define the mechanism(s) by which complement (C') contributes to liver regeneration. The liver has the capacity to regulate its size and growth and to regenerate after partial hepatectomy or toxic injury. Several hormones, pro-inflammatory cytokines, and growth factors have recently been implicated in triggering or regulating this complex homeostatic response through the activation of their respective signaling pathways. The molecular mechanisms by which these factors exert their function are still poorly understood, and further studies are required in order to dissect the early triggering events and define the multiple signals and molecular pathways that synergistically interact to promote normal liver regeneration. The liver is the primary site of biosynthesis of several components of the C' system, which has long been recognized to play a major role in mediating local and systemic inflammatory responses. To date, the potential contribution of C' components to the regulation of liver growth and regeneration has not been investigated. Preliminary experiments conducted in this laboratory have shown that mice deficient in the complement (C') protein C5 exhibit impaired liver regeneration and delayed hepatocyte re-entry into the cell cycle after CC14-induced toxic injury and partial hepatectomy. This proposal consists of two aims: The studies in Aim 1, are designed to define the contribution of C' to normal liver regeneration and dissect the mechanism(s) of its involvement in an in vivo model of partial hepatectomy and a model of CCl4-toxic injury, using mice deficient in complement components as well as various molecular and immunohistochemical techniques. Aim 2 will focus on dissecting the mechanism by which C5 contributes to liver regeneration after partial hepatectomy or toxic injury; the role of other C' components, identified in Aim I to be involved in liver regeneration, will be studied using similar approaches to those outlined below for C5. The experiments in Aim 2 will (i) assess the contribution of a C5a receptor-mediated pathway to liver regeneration, dissect the origin of the C5aR-positive cells that mediate this effect and characterize by mass spectrometric approaches cell surface or cytoplasmic components of the C5aR signaling pathway; (ii) assess the ability of C5 and C5a to modulate the expression IL-6 and TNF-a, cytokines shown to be critical in the regenerative response of the liver; (iii) characterize the effect of C5 on key components of the transcriptional network underlying the early growth response of the liver; (iv) determine whether C5 can regulate cell cycle progression in proliferating hepatocytes; and (v) evaluate the role of C5 in regulating glucose metabolism, a prerequisite for normal liver regeneration. This proposal addresses for the first time the role of C' in liver regeneration. It is designed to dissect critical interactions of C' with cytokine-dependent pathways involved in liver regeneration and should elucidate the novel role of complement in the maintenance of liver homeostasis and the regulation of cell growth.